This invention relates to electro-optical sensor arrays.
Electro-optical sensors are used in many systems where it is required to sense a portion of the electromagnetic spectrum. These systems include fiber-optics, telecommunications devices, electronic cameras, and machine vision equipment, as well as many other commercial and military systems. The electro-optical sensor components that allow these systems to sense electromagnetic radiation can be critical in determining the performance, sensitivity, cost, and dynamic range for the entire system.
Many modern electro-optical sensors contain two primary functional elements: a detector element or array of detector elements, and a read-out circuit. The term xe2x80x9cdetector elementxe2x80x9d is used herein to refer to an individual light detector or to the smallest individual light detecting regions in a detector array. The detector elements receive electromagnetic radiation and convert it into electrical signals. The read-out circuit, frequently an integrated circuit known as a read-out integrated circuit (ROIC), processes the electrical signals produced by one or more detector elements into a signal that is useful for the particular system in which the sensor is employed.
One common type of detector element is the photovoltaic junction detector element. FIG. 1A is a circuit diagram and FIG. 1B is a cross-sectional side view of a typical pn junction photovoltaic detector element 2. In the example of FIG. 1A, detector element 2 is a diode structure including an anode 4 and a cathode 6. A terminal 8 is electrically coupled to anode 4, and a terminal 10 is electrically coupled to cathode 6. Detector element 2 may be fabricated by diffusing a p-type region 12 into an n-type semiconductor 14, thereby forming a pn junction as shown in FIG. 1B. Since detector element 2 is a diode structure that is responsive to illumination, detector element 2 is also called a photodiode.
An electro-optical sensor may be used to spatially sample an electro-magnetic image in discrete sections referred to as pixels (picture elements). The term xe2x80x9cdetector pixelxe2x80x9d is used herein to refer to one or more detector elements electrically coupled to provide a signal corresponding to an individual pixel in an image. In most conventional electro-optical sensors a detector pixel includes only one detector element. For example, the single detector element 2 illustrated in FIG. 1A may be used to sample a single pixel in an image. To sample the image of, for example, a line, the single detector element 2 may be scanned across the line (or the line scanned across the detector element). The electromagnetic radiation received at the detector element 2 is collected sequentially in time as the detector element moves relative to the line.
Alternatively, an image of a line may be sampled (without scanning) with a conventional linear array of detector elements each of which samples a pixel of the image of the line. In typical linear arrays, individual detector elements are fabricated next to each other in close proximity and in the necessary quantity to support the system application. FIG. 2A is a circuit diagram and FIG. 2B is a perspective view of a conventional pn junction photovoltaic detector array 16. In the example of FIG. 2A, detector array 16 includes four detector elements 2, each with terminals 8 and 10. In FIG. 2B, four p-type regions 12 (one for each detector element 2) are shown arranged in a line and diffused into n-type semiconductor 14. A linear array 18 having 72 closely spaced detector elements 2 is illustrated in FIG. 3. Typical linear arrays contain as many as 512 or more of such closely spaced detector elements.
For many applications one-dimensional image sampling with a linear array is adequate to provide the necessary information for the system. Spectrometers are an example of this type of application. For applications requiring two-dimensional image information, the image to be sampled may be scanned across the linear array and sampled sequentially in time to capture the image. Scanning mirrors and scanning mechanisms are typically used to provide this capability.
Sampling of a two-dimensional image without scanning may be accomplished with a conventional two-dimensional array of detector elements (also called a staring array) such as detector array 20 illustrated in FIG. 4. Although array 20 includes 1024 detector elements 2 in a 32 by 32 arrangement, it is typical to find two-dimensional arrays containing as many as 1024xc3x971024 detector elements. Array 20 can acquire a 32 by 32 pixel two-dimensional image without the use of a scanning mirror or scanning mechanism if each detector element samples a pixel of the image.
In electro-optical sensors, each detector element in the detector array is electrically connected to the read-out circuitry. In the case of a detector that has a single detector element 2, as in FIG. 1A, it is reasonable to consider electrically connecting to the detector by means of wires and/or printed circuit board traces. However, in one-dimensional linear arrays, such as arrays 16 (FIGS. 2A and 2B) and 18 (FIG. 3), or in two-dimensional staring arrays, such as array 20 (FIG. 4), it becomes unrealistic to interface to the detectors using these methods. These arrays may contain from 512 detector elements to over one million detector elements, thus requiring from 512 to over one million electrical connections respectively. Furthermore, the detector elements are typically small, having widths of, e.g., 25 xcexcm, and are closely spaced within the array. Thus, for example, 1 by 72 linear array 18 (FIG. 3) may have a total width of less than 2 mm. To allow for the small size of the detector elements and array, it is desirable to electrically interface the detectors directly to the readout circuit. It is also desirable to have the readout circuit elements in close physical and electrical proximity to the detector elements due to noise and manufacturing considerations. To meet these requirements for the electrical connection, integrated circuit wire bonding techniques and bump bonding technologies are employed to electrically connect detector elements to the read-out circuit.
FIG. 5 illustrates an electro-optical sensor 22 including a detector array 24 (linear or two dimensional) interfaced to an ROIC 26 through wire bonding or bump bonding techniques. Detector array 24 is in direct electrical contact with ROIC 26 and signals from each of the detector elements in the array are connected to interface electronics in ROIC 26. Each bump bond connects a detector element to a corresponding set of interface electronics, often called the unit cell, which is located directly under the detector elements. The interface electronics, or unit cell, of the ROIC often provides the functions of biasing the detectors, integrating signal from the detectors, and multiplexing the integrated signals to the periphery of the array and to the system.
ROICs are typically formed in silicon using Complementary Metal Oxide Semiconductor (CMOS) technology. For electro-optical sensors that detect electromagnetic radiation in the visible spectrum and/or in the infrared spectrum up to a wavelength of approximately 1.0 xcexcm, silicon can be used to form the detector as well as the readout circuit. For optical sensor components that operate at significantly shorter or longer wavelengths, alternate detector materials may be selected to provide the appropriate sensitivity for the desired region of the electromagnetic spectrum. For example, electro-optical sensors that detect infrared radiation may employ Indium Gallium Arsenide (InGaAs) or Indium Antimonide (InSb) as the detector materials. In such cases, the material used for the detector may be different from the silicon CMOS material technology that is preferred for use in the ROIC.
A problem with electro-optical sensors is that defects that can degrade sensor performance may occur in the detector elements. These defects occur as a result of the materials or manufacturing processes used to form the detector elements. For example, detector materials such as Indium Gallium Arsenide (InGaAs) often contain defects that manifest themselves as regions where the n and p layers are short-circuited together. Such defects may arise, for example, from the lattice mismatch between the Indium Phosphide (InP) substrate and the InGaAs material deposited on top. Detector defects may also occur, for example, during photolithographic or metallization steps in the manufacturing process. Defects in the detector elements are typically very small compared to the size of the detector elements and are randomly distributed throughout the sensor. Generally, non-silicon based detector materials, such as InGaAs, have a much higher defect density than silicon based detectors.
One of the more detrimental effects of detector defects is that the defects can alter the bias voltage versus current (I-V) characteristics of a detector element. In particular, defects that short-circuit a photovoltaic detector element make the detector element""s I-V characteristics more ohmic depending on the severity of the defects. For example, FIG. 6 shows I-V curves 28, 30, and 32 representative of photovoltaic detector elements having, respectively, normal (no defects), poor, and ohmic I-V characteristics. The right and left halves of FIG. 6 are typically referred to as the forward bias region (FB) and the reverse bias region (RB), respectively. Under forward bias, a photovoltaic detector""s zero current intercept, or forward voltage, is a function of the illumination level. Similarly, under reverse bias the reverse bias current is a function of the illumination level. In addition, the reverse bias current contains a junction leakage current component and, under high reverse bias conditions, a reverse bias breakdown current component. The magnitude of the leakage current and/or reverse bias break down current may be large compared to the detector photocurrent and typically depends on the properties of the detector material. In particular, material and manufacturing defects may degrade the detector reverse bias current performance.
For each of the detector elements represented by I-V curves 28, 30, and 32 a unique offset reverse bias current 34, 36, and 38, respectively, develops as a result of reverse biasing the detector elements at, for example, VRB. These offset currents, which depend on the number and severity of defects in the detector element, have the effect of introducing variations in the output signals for the detector elements. In some cases these variations can represent a significant portion of the dynamic range for the detector element signal levels. In addition, the defects can also degrade the noise performance of the detector elements.
In most conventional electro-optical sensor arrays the detector elements interconnect to the ROIC signal processing electronics and/or multiplexing circuitry directly. In these devices the signals from the detector elements are integrated and multiplexed irrespective of the quality of the detector element. For example, a defective detector element that provides an abnormally high or an abnormally low detector current would have its signal multiplexed in the output of the ROIC irrespective of these anomalies. Consequently, the existence of defective detector elements in conventional detector arrays can be a serious problem if the array is used in an application that requires a high level of performance. For example, in some telecommunications systems a conventional linear array of detector elements may be used to monitor the power of individual channels of a multi-channel Dense Wavelength Division Multiplexing (WDM or DWDM) optical signal. This application may require that none of the detector pixels in the array be defective if all of the optical channels are to be monitored accurately.
In many cases detector arrays including detector elements that generate anomalous signal levels may be classified as defective arrays due to the anomalous detector elements. Moreover, the signals from the anomalous detector elements may act to alter or corrupt signals from neighboring detector elements, further reducing the likelihood for the array to meet acceptable performance levels. Hence, defects in detector elements reduce the manufacturing yield of conventional electro-optical sensor arrays.
The paper xe2x80x9cOptimizing scanning array performance using gain normalization and time delay and integrate (TDI) pixel deselection during readout, hybrid and focal plane testingxe2x80x9d by Darryl Adams, Greg Johnson, Noel Jolivet, Jeff Metschuleit, SPIE Vol. 1686, Test and Evaluation of IR Detectors and Arrays 1992, describes an electro-optical sensor array in which defective detector pixels are deselected prior to processing of their signals by a ROIC multiplexer and output electronics circuitry. The time delay and integrate circuitry required by this electro-optical sensor makes the sensor much more complex and thus typically more expensive than it would be otherwise. Moreover, the method described in this paper can be applied only to scanning arrays, not to staring arrays.
An electro-optical sensor in accordance with the present invention includes a detector pixel including a plurality of detector elements responsive to electromagnetic radiation, and a plurality of switches configurable to selectively combine signals from the detector elements in the detector pixel to provide a pixel signal corresponding to a pixel in an image. The detector elements may be selected or deselected to contribute to the pixel signal either individually or in groups.
In one embodiment, for example, one of the switches is configurable to either include in or exclude from the pixel signal the signals provided by a group of two or more detector elements. In another embodiment, one of the switches is configurable to either include in or exclude from the pixel signal a signal provided by an individual detector element. The sensor may also include an amplifier coupled to receive the pixel signal. The gain of this amplifier may be adjustable to compensate for exclusion from the pixel signal of one or more signals provided by detector elements in the detector pixel.
Electro-optical sensors in accordance with the present invention may include a plurality of such detector pixels arranged, for example, in a linear array or in a two-dimensional array. Each of the detector pixels may have an associated group of switches configurable to selectively combine signals from the detector elements in the detector pixel.
In a method for configuring an electro-optical sensor in accordance with the present invention, detector elements included in a detector pixel are selectively electrically coupled to provide a signal corresponding to a pixel in an image. In one embodiment, the performance of detector pixels or groups of detector pixels is tested. Detector elements or groups of detector elements found to be defective may be selectively decoupled such that they do not contribute to the pixel signal.
The ability to select operable and deselect defective detector elements from detector pixels in the inventive electro-optical sensors allows such sensors to include defective detector elements and yet have all detector pixels operable. Consequently, electro-optical sensors in accordance with the present invention may be manufactured with full operability in much higher yield than is possible with prior art sensors.